Mill

ABSTRACT

Various pieces of raw or scrap plastic, foods, pharmaceuticals, chemicals both dry and wet, materials are fed into the mill and reduced in size into particles in a size range of from 1 to 3 microns to larger micron sizes. The rotor has a plurality of rows of blades mounted thereon which serve to reduce the fed materials into the smaller sizes. The screen below the rotor permits the exiting of the desired sizes from the mill.

I Un ted States Patent [1 1 3,680,797 Covey 1 51 Aug. 1, 1972 15 1 MILL 2,785,865 3/1957 Berling ..241/191x 72 I t Go d C H 3,170,640 2/1965 Kolts ..241/86 1 g gg 3" 5 Glendale Rd 2,440,051 4/1948 Lind ..241/191 x Filed: 1969 Primary Examiner-Robert C; Riordon Assistant Examiner-Gary L. Smith [21] Appl' 880826 Attorney-Kenyon 8L Kenyon Reilly Carr & Chapin [52] U.S. Cl ..241/73, 241/191 [57] ABSTRACT [51] Int. Cl. ..B02c 13/06, B02c 13/28 f 58 Field ofSearch...24l/86, 88, 73, 191; 308/361, vamus. peces J z 308/132 maceutlcals, chemicals both dry and wet, materlals are fed into the mill and reduced in size into particles in a size range of from 1 to 3 microns to larger micron [56] References Cited sizes. The rotor has a plurality of rows of blades UNITED STATES PATENTS mounted thereon which serve to reduce the fed materials into the smaller sizes. The screen below the 32E 5 rotor permits the exiting of the desired sizes from the e "1L 1,890,844 12/1932 Delaval-Crow ..308/132 X m 2,385,767 9/1945 Wagner ..241/88 X 15 Claims, 8 Drawing Figures PATENTEDAUE 1 m2 .SHEET 2 [IF 5 GORDON W. COVEY PATENTEDMM; 1 m2 3 D 6 8 O. 7 9 7 sum 5 or 5 1n; GORDON WCOVEV MILL This invention relates toa mill. More particularly, this invention relates to a mill for granulating, fine grinding, slurry grinding pieces of material.

Heretofore, various types of mills have been utilized in the granulation of plastic materials into fine sizes.

For example, such mills have been constructed so as tov direct the raw material to be granulated into a chamber in which a rotating rotor is disposed with a plurality of blades thereon. The rotor and blades have been positioned within the housing so that the material is caused to flow circumferentially about the rotor during rotation of the rotor under centrifugal force while the blades simultaneously sever .the raw material into smaller sizes. These heretofore mills, however, have relied on a saw tooth operation such that the resulting product has had a wide gradation of ground or granulated sizes. Accordingly, it has been necessary to further process the resulting granulated material to a sieving operation to separate the various sizes of material. In addition, the mills have usually used blades which have been so connected to the rotors as to require considerable time and effort for replacement. In some instances, where the mounting of the blades have been of a substantially permanent nature, it has been necessary to disassemble the mill and mount another rotor with a different set of blades thereon when different sizes or gradations of granulated materials were required from a given material. Similar procedures have also. been necessary where the raw material changes in composition.

Accordingly, it is an object of this invention to granulate micron sized particles or larger from a batch of raw materials in a simple efficient manner.

It is another object of this invention to granulate raw plastic materials into predetermined micron sizes.

It is another object of this invention to granulate raw plastic materials into micron sizes in a single operation.

It is another object of this invention to vary the granulatedsize of particles by changing the speed of a rotor.

It is another object of this invention to provide a mill which can be easily cleaned.

Briefly, the invention provides a mill for granulating materials into micron size or larger. The mill includes a housing having a discharge opening, a feed hopper for the introduction of the plastic material into the housing and a rotor having a plurality of blades mounted thereon within the housing for severing and reducing the fed materials to micron size or larger. The housing further has a cover which is mounted so as to enclose the rotor and to cooperate with the feed hopper so as to direct the fed material into the path of the blades on the rotor. Additionally, a screen is mounted within the housing about the lowerend of the rotor in spaced relation to the radial extent of the blades. The screen is sized with predetermined openings to define the size of the desired particles so that after reduction of the fed material to size, the particles can pass through the screen out of the discharge opening.

In use, the raw materials are fed through the feed hopper into the confines of the housing so as to be granulated by the action of the blades upon rotation of the rotor. As the pieces of material are being constantly reduced in size due to the severing action of the individual blades, the material approaches a micron size sufl'rcient to pass through the openings of the screen downwardly through the discharge opening of the housing. The reduction of the raw material in size is accomplished by the blades due to the centrifugal force of the rotor which forces the larger pieces of the raw material radially outwardly of the rotor and the lag in speed of the plastic particles relative to the speed of the blades. This continues until the particles have been reduced in size sufficient to pass through the openings of the screen.

It is noted that the rotor is of a substantial size relative to the blades such that a relatively large number of rows of blades can be mounted on the rotor. This permits a greater efficiency to be achieved since more cutting edges are utilized for each revolution of the rotor. The increased size of the rotor is possible since the centrifugal force on the material causes the material to confine itself to the outer limits of the housing. Thus, the rotor can be sized to occupy substantially all the space'which would not be occupied by the material without reducing the capacity of the mill.

The blades which are used within the mill are formed with cutting edges along two opposite sidewalls and the top such that the blades can be used to sever the material from either direction of rotation of the rotor. This is an advantage should the rotor be driven in one direction for an extended period of time, since the direction of rotation of the rotor can be reversed to sever the material without tearing down the mill for replacement of the blades. In addition, the blades are mounted in a plurality of rows about the periphery of the rotor. For example, the blades are mounted on suitable knife bars which are slidablymounted and locked inplace in the rotor. Each knife bar serves to carry a plurality of blades thereon in a fixed secured manner. The blades are further disposed in a predetermined pattern with respect to adjacent rows so as to determine the size of the particles which are cut and granulated by the mill.

These and other objects and advantages of the invention will become more apparent from the following detailed description and the accompanying drawings in which:

FIG. 1 illustrates a perspective view of a mill according to the invention;

FIG. 2 illustrates a cross-sectional view of the mill of FIG. 1;

FIG. 3 illustrates a perspective view of the mill of FIG. 1 in an open condition;

FIG. 4 illustrates a fragmentary cross-sectional view of a rotor and knife bar arrangement with respect to the screen of the mill;

FIG. 5 illustrates a top view of the knife bar and blades of FIG. 4;

FIG. 6 illustrates a fragmentary exploded view of a knife bar and blade arrangement for mounting on the rotor;

FIG. 7 illustrates the mounting of a blade on the knife bar; and

FIG. 8 illustrates a developed view of a pattern of the blades spaced according to the invention.

Referring to FIG. 1, the mill 10 is constructed so as to be utilized with a suitable conveyor device 11 for feeding material to be granulated thereinto and a suitable discharge means 12 such as a chute for expelling material which has been granulated therefrom. The mill includes a housing 13 which is secured to a mounting frame 14, for example, of rectangular construction, so as to be disposed over a chute 12 or other suitable discharge means. The housing 13 is constructed with a base 15 which is secured directly to the mounting frame 14 by pairs of legs 16 which extend outwardly of the remainder of the base 15. Each leg 16 has a mounting pad 17 thereon which is secured, as by bolts 18, to similar pads 19 fixed on the mounting frame 14. Suitable resilient or elastomeric blocks 20 are disposed between the respective pads 17, 19 of the base 15 and mounting frame 14 so as to compensate for vibrational forces or strains imposed on the housing 13. The housing 13 further includes a cover 21 which is pivotally mounted as by a pair of throat hinges 22 on the base 15. Two opposite sides of the base 15 are provided with hinge mountings 23 for the throat hinges 22 so that the cover 21 can be oriented on the base 15 in either one of the two positions as required. Additionally, the cover 21 is provided with a number of locking mechanism 24 which are disposed around the sides and front of the cover 21 so as to be secured to the base 15. Each locking mechanism 24 includes a cylinder 25 which is integrally secured to an outwardly extending flange 26 of the cover 21, a shaft 27 which is rotatably mounted within the cylinder 25 and which carries a handle 28 at the upper exposed end thereof for rotation of the shaft 27 relative to the cylinder 25 as well as a bar 28 at the lower end which extends from the shaft 27 so as to be rotated therewith. The shaft 27 is sized so that the bar 28 is disposed below the plane of a flange 29 of the base 15 such that the bar 28 can be rotated into a locking position under the flange 29 of the base 15 (FIG. 2). The cover 21 is also provided with pairs of lifting bars 30 which extend from the opposite sidewalls thereof. These lifting bars 30 can be of any suitable construction and are used to facilitate the pivoting of the cover 21 with respect to the base 15 or to facilitate lifting of the housing 13 or mill 10 into position.

Referring to FIG. 2, the base 15 of the housing 13 is of substantially rectangular configuration and is open at the lower end so as to form a discharge opening 31 which communicates with a chute skirt 32 so that granulated material can be discharged therethrough. The cover 21 is substantially of semi-cylindrical crosssectional shape and is provided with a throat portion 33 near the upper end. This throat portion 33 is substantially angularly disposed relative to the interior of the housing 13 and is provided with a peripheral flange 34 or a lip at the upper end which extends outwardly thereof.

A feed hopper 35 is mounted on the flange 34 of the throat portion 33 of the housing 13 in order to direct a charge of material into the throat portion 33. This feed hopper 35 includes a flange 36 at the lower end which seats on the flange 34 of the cover 21 and is locked thereto as by a plurality of locking mechanisms 37 as described above. As shown, the locking mechanisms 37 are integrally secured about the sides of the feed hopper 35. The feed hopper 35 further comprises a hollow section of substantially rectangular configuration which extends upwardly of the cover 21 of the housing and includes an opening at the outer end which is disposed in a substantially vertical plane so as to receive a charge of material from a substantially horizontally disposed conveyor 38.

In order to facilitate charging of the mill 10, the feed hopper 35 is provided with a feed conveyor shaft 39 upon which a roller 40 is mounted for rotation. This roller 40 cooperates with an endless conveyor belt 41 (FIG. 1) which serves to feed a charge of material over the roller 40 into the feed hopper 35. Alternatively, a screw conveyor could be used with feed rolls on centerline.

Referring to FIGS. 1 and 3, in order to granulate the material fed into the hopper 35, a one-piece rotor 42 is joumalled in a pair of bearings 43 mounted on opposite sides of the mounting frame 14 and a plurality of rows of blades 44 are mounted in a predetermined pattern on the rotor 42. The rotor 42 has a pair of stub shaft ends which are rotated in the bearings 43 as well as an enlarged central portion which carries the blades 44 within the housing. Each stub shaft has a keyway 45 which serves to connect the rotor 42 to a suitable drive for rotation of the rotor 42. Each bearing 43 is of known construction and is provided with an oil mist lubrication via a line 46 as is known. In order to secure the blades 44 in place, the enlarged portion of the rotor 42 has a plurality of circumferentially spaced slots of trapezoidal cross-section which slidably receive a plurality of longitudinally elongated knife bars 47 therein. Each knife bar 47 carries a plurality of blades 44 in fixed relation.

Referring to FIGS. 2, 4 and 6, each knife bar 47 is mounted in a removable manner within a slot 48 within the periphery of the rotor 42 so that a number of circumferentially spaced rows of blades 44 can be mounted on the rotor 42. To this end, each slot 48 in the rotor 42 is of substantially trapezodial cross section and is open at the top and ends while each knife bar 47 is of a mating configuration to slide into the slot 48 from either end of the rotor 42. In order to secure each knife bar 47 in place, pairs of holes 49 are formed transversely through the bar 47 to match up with corresponding threaded holes 50 in the rotor 42 within the slots 48 so that locking bolts 51 can be passed through the holes in the bar into threaded engagement in the rotor 42 to hold the knife bar 47 in place. As shown, a pair of knife bars 47 are mounted symmetrically within each slot 48.

Alternatively, the knife bars 47 can be held within the slots 48 by locking screws (not shown) which are threaded into and through the bars 47 so as to abut against the floor of the slots 48. In this way, the locking screws serve to lift the bars 47 slightly against the tapered sidewalls of the slots 48. This allows for ease of removal of the bar without tools and enables the bar to be turned end for end, this presenting a new blade edge to the product.

Each knife bar 47 includes a pair of locking wedge surfaces 52 which are slidable within the corresponding surfaces of a slot 48 of the rotor 42 and an upstanding attachment portion 53 which extends upwardly therefrom to include a flat top surface 54 which functions of a continuation of the peripheral rotor surface 55. The attachment portion 53 assures that there is a continuity of surface so that particles of material are prevented from becoming trapped between the knife bar 47 and rotor 42.

The attachment portion 53 of each knife bar 47 is provided with a plurality of transverse slots 57 of sub stantially trapezodial cross section. Each of these slots 57 is sized to receive a knife shank 58 therein in slidable relation. Each knife shank 58 has a lower trapezodially shaped portion 59 which is slidably mounted within each of the slots 57 and an upwardly projecting attachment portion 60. This latter attachment portion 60 receives a blade 44 thereon in fixed relation. To this end, the attachment portion 60 is provided with a pair of openings 61 through which a pair of threaded bolts 62 are passed within suitably threaded openings 63 in the lower end of a blade 44. In order to secure the knife shank 58 within the slot 57 of the knife bar 64, a pair of set screws 65 are mounted in each of the openings 64 from below. After each knife shank 58 is slid into a slot 57, the set screws 65 are threaded from below into abutment with the bottom surface of the knife shank 58 so as to lift the knife shank 58 slightly against the mating surfaces of the slot 57 into fixed securement.

Referring to FIG. 2, each knife blade 44 is of substantially rectangular configuration and extends outwardly of the knife bar 47 substantially radially of the rotor 42. To this end, the blades 44 extend from the outer periphery of the rotor 42 a distance equal to from one-quarter to one-half the radius of the rotor 42. As shown, each knife blade 44 is provided with a cutting edge 66 on two opposite sides and along the top edge. These surfaces can be formed by tapering one side of the blade along each of the respective edges. The blades are thus capable of cutting when rotated in either direction of rotation of the rotor. Alternatively, as shown in FIG. '7, each knife blade 44 can have a blunt cutting edge 67 as is known. Also, a knife blade can be constructed which is integral with a shank portion which serves to mount the blade on the knife bars 47. Such a blade can be used for impact cutting.

Referring to FIG. 8, the various blades 44 which are mounted on the several knife bars 47 in the rotor 42 are disposed in a predetermined pattern such that an efficient severing or granulation of the charged material to the predetermined micron size or larger can be carried out. For example, where six rows of blades 44 are used, each row containing two knife bars of symmetrical construction about the central transverse plane of the rotor, the blades 44 are each mounted on spacings of 2 inches between blades on each bar while the knife shanks 58 are mounted on center lines which are spaced from the center line of the blade pattern by distances of 1 5/16; 2 3/l6; l 5 1; 2 21/32; "/8; and 27/64 inches, respectively. In such cases, the attachment portion 60 of each knife shank 58 has a thickness of approximately 2.309 inches while the knife blades have their cutting edges substantially in abutment with the side surfaces of the attachment portions 60. A typical spacing is shown in FIG. 4.

In addition, a stuffing box seal 68 is disposed at the outer sidewalls of the housing 13 to seal off the interior of the housing to the exterior about the rotor surfaces. As shown, the stuffing box seal 68 serves to effectively seal off any space between the rotor 42 and the semicylindrical portions in the cover 21 and base of the housing 13 so that the oil mist lubrication in the bearings 43 is protected against the heat inside the housing.

Referring to FIGS. 2 and 3, the housing base 15 is further provided with a screen 69 of semi-cylindrical shape which is disposed below the plane of the enlarged portion of the rotor 42 within the housing 13. The screen 69 is sized so as to be spaced from the path of the blades 44 with a slight clearance such as one-fourth inch and functions to permit granulated material to flow through the openings therein down through the discharge opening of the base 15. The opposite ends 70, 71 of the screen 69 are tapered outwardly at a slight angle from the remainder of the screen and fit within suitable recess portions 72, 73 at the upper ends of the base 15. The cover 21, on the other hand, is provided with a shoulder 74 on one side which receives and abuts against one end 70 of the screen 69 as well as with a depending lip 75 on opposite sides to overlie the screen ends 70, 71. In this way, the cover 21 of the housing 13 serves to secure the screen 69 in place against movement longitudinally thereof under the forces generated within the housing 13 by the rotating blades 44 and material.

The screen is provided with a plurality of rows of openings 76 of suitable size so as to permit the passage of a predetermined size of granulated particles therethrough. As the screen 69 is easily removable, a plurality of screens, each with differently sized openings, can be employed with the mill to obtain different sizes of granulated material.

In operation, (FIG. 1) a charge of material such as vinyl floor tile, Teflon, or other plastic type material, in pieces of substantial dimensions is fed into the feed hopper 35 by way of the feed conveyor shaft 40. The material then drops down into the interior of the housing 13 through the throat section portion 33 of the cover 21. During this time, the rotor 42 is rotating by means of the drive means (not shown) in a clockwise direction as viewed in FIG. 2. The pieces of material upon coming into the plane of the blades 44 of the rotor 42 are then caused to rotate along with the blades 44 in the spaces between the rows of blades while being confined between the cover 21 and the rotor 42. The material thus becomes pushed along under the centrifugal force of the blades 44 as well as by the continued feeding of material into the hopper 35. In addition, since the pieces of material are subjected to certain inertia and drag forces from the walls of the housing cover 21 and screen 69, the speed of the pieces is less than the speed of the blades 44. As a result, the blades 44 upon passing by the pieces of material sever the pieces into smaller pieces. The process continues as the material is conveyed around the outside of the rotor 42. Continued rotation of the rotor 42 and continued movement of the blades 44 relative to the pieces of material causes subsequent severing of the pieces into smaller and smaller sizes. Upon reaching a size of particle which is equal to or less than the size of the openings 76 in the screen 69, the granulated particles of material drop through the screen 69 and pass through the discharge opening in the bottom of the housing 13.

It is noted that for a rotor having a diameter of 23 inches and a working width of 44 inches within the housing and being rotated off a drive of approximately 200 horsepower and having blades of about 4 inches in length extending from the rotor periphery, a rate of granulation can be carried out of approximately 20 thousand pounds of hot mix or 12 thousand pounds of cold mix of a vinyl material in an 8 hour working day.

In order to provide the mill 10 with a means for indicating closure of the cover 21 on the base 15, a switch 80 (FIG. 3) is mounted on the base so as to be activated by the flange 29 of the cover 21 upon closure of the cover 21 and locking thereof on the base 15. The switch 80 is constructed, for example, with a depressable button 81 which projects into the plane of the flange of the cover 21 so as to be depressed thereby. The button 81 actuates the switch 80 upon being depressed so that a suitable signal can be transmitted from the switch 80 to a suitable receiver. For example, the receiver can be constituted by a warning light (not shown) on the mill 10 so as to indicate when the cover 21 is securely locked in place such that the mill can be placed in operation.

It is noted that instead of using a single knife or mounting a single blade, a knife shank can be used for mounting of two blades in each slot of a knife bar. To this end, each knife shank is formed with a trapezodially shaped lower portion and a pair of attachment portions which extend upwardly therefrom in spaced relation to each other. Each of these attachment portions would mount a blade to the outside thereof so as to achieve the proper spacing.

It is further noted that the mill can be used to granulate any suitable material such as vinyl floor tile, Teflon, virgin Teflon, or any other type of plastic material.

It is also noted that the cutting blades which are utilized in the'mill can be provided with sharp cutting edges for example, for use with the Teflon or can be provided with cutting edges which are substantially flat i.e. of the square impact type, for other types of materials. Also, the cutting surfaces of the blades can be boron coated so as to be more resistant to wear and abraision.

The invention thus provides a mill in which the size of the material being granulated is determined by the size of the rotor, the speed of the rotor and the number of blades ejecting from the rotor. In this regard, a multiple pass is used for materials such as Teflon to achieve a decreasing size. For example, in order to obtain Teflon particles of l to 2 micron average with a maximum size of 15 microns, the screen has openings therein of from 0.006 to 0.008 inches.

The invention thus provides a mill which is useful in obtaining particle sizes of very small dimension and uniformity. For example, Teflon pieces can be granulated to a size such as 150 microns for use in spray dispersions.

In addition, the mill is able to granulate the fed material into the desired sizes in a rapid manner without any significant waste of space within the interior of the housing. Thus, a more efficient mill is obtained.

The material can be fed into the mill in any suitable manner such as by a feed conveyor belt, a screw fed means, or manually.

In the case of a vinyl material, such can be introduced into the mill while still in a hot mix, for example, directly from a Banberry mixer. In such a case, the feed would be in the form of a hot blanket which upon processing in the mill is granulated into a suitable size, for example, for Terrazo tile.

Further, the invention provides a mill which can carry out a granulation process and which can be readily adapted to provide different sizes of particles, for example, by changing the speed or size of the blades. In this latter regard, the blades can be rapidly removed and replaced in the mill so as to accommodate different materials and sizes without removing the rotor from the housing.

It is noted also that since the chute feed of the mill is reversable, the rotor can also be mounted in a reversible manner or rotated in a reversible manner so as to compensate for any reorientation of the feed hopper.

What is claimed is:

1. A mill for granulating material comprising a housing having an inlet opening to receive the material and a discharge opening to discharge the material in a granulated size;

a rotor rotatably mounted in said housing between said openings, said rotor having a pair of reduced diameter stub shaft ends joumalling said rotor in said housing and an enlarged central portion having a plurality of circumferentially spaced slots therein;

a plurality of knife bars, each of said knife bars being slidably mounted in a respective one of said slots;

a plurality of blades mounted on and extending from each said knife bar in a plurality of peripherally spaced rows and in a staggered pattern with respect to each other about said rotor and extending radially outwardly from the outer periphery of said rotor a distance equal to from one-quarter to one-half the radius of said rotor for granulating the received material upon rotation of said rotor, said blades in each row being disposed in symmetrical relation about a central transverse plane of said rotor;

means for releaseably locking said knife bars in said slots; and

a screen mounted in said housing about the lower side of said rotor in spaced relation to said blades, said screen having a plurality of openings of predetermined size therein to permit passage of granulated material of at least said predetermined size.

2. A mill as set forth in claim 1 wherein said housing includes a frame portion supporting said rotor and said screen thereon and a cover mounted on said frame portion over said rotor and said screen.

3. A mill as set forth in claim 2 wherein said cover is pivotally mounted on said frame portion to pivot away from said rotor and screen.

4. A mill as set forth in claim 2 wherein said cover includes a pair of lips disposed on opposite sides, said lips projecting outwardly of said cover into the plane of said frame portion to grip two ends of said screen between said cover and said frame portion to prevent displacement of said screen.

5. A mill as set forth in claim 2 wherein said housing further includes a feed hopper mounted on said cover for directing the received material towards said rotor.

6. A mill as set forth in claim 5 wherein said feed hopper is removably mounted on said cover.

7. A mill as set forth in claim 1 which further comprises bearing means mounted on said housing rotatably mounting said rotor thereon.

8. A mill as set forth in claim 7 which further comprises oil mist lubrication means connected to said bearing means for lubricating said bearing means.

9. A mill as set forth in claim 7 which further comprises stuffing box seal on said rotor adjacent said housing for sealing said rotor with respect to said housing.

10. A mill as set forth in claim 1 which further comprises a mounting frame secured to said housing for mounting of .said discharge opening over a predeter-, mined space.

11. A mill as set forth in claim 1 wherein said screen has a plurality of openings therein of a size of about microns.

,12. A mill as set forth in claim 1 wherein said screen is substantially semi-circular and is spaced from the radial extent of said blades.

13. A mill as set forth in claim 1 wherein said blades on said knife bars are arranged in a predetermined pattern to obtain predetermined sizes of granulated particles.

14. A mill as set forth in claim 1 wherein each slot is of trapezoidal cross-section.

15. A mill as set forth in claim 1 which includes a plurality of knife shanks secured on each knife bar, each said shank having a respective one of said blades removably mounted thereon. 

1. A mill for granulating material comprising a housing having an inlet opening to receive the material and a discharge opening to discharge the material in a granulated size; a rotor rotatably mounted in said housing between said openings, said rotor having a pair of reduced diameter stub shaft ends journalling said rotor in said housing and an enlarged central portion having a plurality of circumferentially spaced slots therein; a plurality of knife bars, each of said knife bars being slidably mounted in a respective one of said slots; a plurality of blades mounted on and extending from each said knife bar in a plurality of peripherally spaced rows and in a staggered pattern with respect to each other about said rotor and extending radially outwardly from the outer periphery of said rotor a distance equal to from one-quarter to one-half the radius of said rotor for granulating the received material upon rotation of said rotor, said blades in each row being disposed in symmetrical relation about a central transverse plane of said rotor; means for releaseably locking said knife bars in said slots; and a screen mounted in said housing about the lower side of said rotor in spaced relation to said blades, said screen having a plurality of openings of predetermined size therein to permit passage of granulated material of at least said predetermined size.
 2. A mill as set forth in claim 1 wherein said housing includes a frame portion supporting said rotor and said screen thereon and a cover mounted on said frame portion over said rotor and said screen.
 3. A mill as set forth in claim 2 wherein said cover is pivotally mounted on said frame portion to pivot away from said rotor and screen.
 4. A mill as set forth in claim 2 wherein said cover includes a pair of lips disposed on opposite sides, said lips projecting outwardly of said cover into the plane of said frame portion to grip two ends of said screen between said cover and said frame portion to prevent displacement of said screen.
 5. A mill as set forth in claim 2 wherein said housing further includes a feed hopper mounted on said cover for directing the received material towards said rotor.
 6. A mill as set forth in claim 5 wherein said feed hopper is removably mounted on said cover.
 7. A mill as set forth in claim 1 which further comprises bearing means mounted on said housing rotatably mounting said rotor thereon.
 8. A mill as set forth in claim 7 which further comprises oil mist lubrication means connected to said bearing means for lubricating said bearing means.
 9. A mill as set forth in claim 7 which furTher comprises stuffing box seal on said rotor adjacent said housing for sealing said rotor with respect to said housing.
 10. A mill as set forth in claim 1 which further comprises a mounting frame secured to said housing for mounting of said discharge opening over a predetermined space.
 11. A mill as set forth in claim 1 wherein said screen has a plurality of openings therein of a size of about 15 microns.
 12. A mill as set forth in claim 1 wherein said screen is substantially semi-circular and is spaced from the radial extent of said blades.
 13. A mill as set forth in claim 1 wherein said blades on said knife bars are arranged in a predetermined pattern to obtain predetermined sizes of granulated particles.
 14. A mill as set forth in claim 1 wherein each slot is of trapezoidal cross-section.
 15. A mill as set forth in claim 1 which includes a plurality of knife shanks secured on each knife bar, each said shank having a respective one of said blades removably mounted thereon. 